Midblock sulfonated block copolymers are known. Typically, they are sulfonated polymers based on styrene and/or t-butyl styrene with the former predominantly used in a midblock, that is subsequently sulfonated and the latter in the endblocks, that resist sulfonation. These polymers are in a solid state in the presence of water and have both high water transport properties and sufficient wet strength. These polymers are known to have excellent barrier properties.
From WO2007010039 a midblock sulfonated styrenic block copolymer is known. This block copolymer is based on a block copolymer that comprises at least two polymer end blocks A and at least one polymer interior block B wherein each A block is a polymer block resistant to sulfonation and each B block is a polymer block susceptible to sulfonation, and wherein said A and B blocks do not contain any significant levels of olefinic unsaturation.
Such polymers are now commercially available for instance under the trademark Nexar® from Kraton Polymers. The typical structure of a Nexar molecule is a pentablock consisting of two poly(t-butylstyrene) (tBS) blocks, two poly(ethylene/propylene) (EP) blocks (hydrogenated polyisoprene), and in the middle a partly sulfonated polystyrene (sPS) block.
Such midblock sulfonated block copolymers are typically delivered to customers as a solution of about 10% in a combination of heptane and cyclohexane. For some customers this poses a problem because they are not used to handling this type of solvent and do not have adequate ventilation and disposal systems in place. Organic solvents may cause various handling problems due to the high volatility and low flame point of such solvents. The possibility to deliver such polymers as an aqueous emulsion would be a solution. Moreover, a waterborne system is more environmental friendly. Preparing a suitable aqueous emulsion, however, is not without its own problems.
EP2242137 and EP1852928 concern a membrane-electrode assembly for polymer electrolyte fuel cells. They employ a block copolymer comprising a polymer block (A) having ion-conductive groups and a polymer block (B) having no ion-conductive groups, both polymer blocks phase-separated from each other, polymer block (a) forms a continuous phase. In paragraph [0047] of EP2242137 it describes methods of emulsifying the block copolymer. This is described and illustrated for end-block sulfonated block copolymers only. End-block sulfonated block copolymers behave differently from the midblock sulfonated block copolymers. A method for preparing an aqueous dispersion of a midblock sulfonated block copolymer is therefore not disclosed in EP2242137 or EP1852928.
The solution inversion emulsification method, when applied on a midblock sulfonated styrenic block copolymer dissolved in a hydrocarbon solvent like cyclohexane/heptane, produces a rather coarse aqueous emulsion with relatively big average particle size of about 7.0 μm or larger (as determined by laser diffraction spectroscopy). Ideally, the average particle size should be about 2.0 μm or smaller. Smaller particles have better film forming properties.
Using a series of homogenizers is not attractive. The homogenizers are a substantial capital investment. Moreover, the properties of the polymer are affected by the physical homogenization. Furthermore, there is a serious risk of loss of material when mechanically reducing the size of the particles from coarse (7.0 μm or greater) to fine (2.0 μm or smaller). Finally, the pH of the so produced emulsions is very low (<2). A corrosive emulsion may adversely affect the equipment used.
Ideally it should be possible with ordinary equipment to produce emulsions with reduced average particle size (well) below 7.0 μm. This should be possible, even when using a hydrocarbon solvent wherein the preceding polymerization has been performed. Expressed differently, when using a typical solution of a midblock sulfonated styrenic block copolymer dissolved in a hydrocarbon solvent such as cyclohexane, heptane or a mixture thereof, it would be ideal if the average particle size could be reduced by a factor of 4 or greater without having to invest heavily in equipment or having to replace the solvent.